Okay - I'm about to start a new outdoor kitchen. I'm thinking plywood for the support of my countertops covered with granite. Especially around the grill area itself - I'm not seeking a permit and have no uses for local fire codes or "use" ordinances... I'm also contemplating hardiboard as an alternative.

There are those that say plywood is stronger and therefore a better structural element to support and add rigidity to the fragile granite. But it is a flammable material, absorbs moisture and is prone to rot and insect infestation. On the other hand, hardiboard has none of these attributes. But manufacturers claim it is NOT for outdoor use, and should not be considered a structural entity. Woe, alas and whatever shall I do? How do I make the correct decision? Based on centuries of use - wood seems to be a proven winner here. I know I can purchase plywood with "non-burn" features. I have no termite residences in the neighborhood and I'm in a dry climate. So why not?

But technology moves forward. Hardiboard has come into it's own. I know it's moisture repellant. In a recent conversation with my Local 452 Insect labor representative - he claimed no members of his union care for the taste of hardiboard. I heard claims that if it is glued in place and has enough metal stud support - it does add structural support. Yikes! Where do I go for the proper advice? Who is qualified to help me make this decision?

Okay - you all realize some of the above sentences are not exactly serious and factual in content. Bordering on comical contemplation, I offer this query hoping for some qualified, accurate and concise opinions (as this is JUST a forum), and I'm certainly not signing a Contract here with the posters to build this kitchen for me. So I'm expecting the answers given to be opinions, but hopefully views based on some fact, experience and data given the qualifications of the participants.

Plywood? Hardiboard? Or do I have other options? Which is best? And why? Or why not? It seems both work, and the cost is relatively comparable. Help me here? Anyone?

Let the debate begin! And I offer that we keep this congenial. No flaming, barbs thrown or attacks against opposing viewpoints or practical applications in use by other Contractors. Thanks in advance for a mature and harmonious debate!

Nice Thread and some interesting questions Mr. Homeowner/BBQ Island Erector, Might I suggest a fire rated gypsum applied to both sides of your framing members wood or metal framing (your choice) since your island height is not a factor I don't think a structural shear panel will be necessary, I would go 16"s OC and maybe a doubler for your corners for your framing as far as 3/4 plywood as an under lament for your 5/8's to 3/4's granite that is a standard building practice as Hardie Board flexes horizontally unless you'd like to considerably beef up your upper cross members from wall to wall to take the load of the granite for lack of the load bearing properties found in the Hardie board products and also if you use a creosote sealer termites will run from the plywood, interior wise you have butted your plywood slash/granite installs right up to stove tops, have you not Sir? or wrapped an oven in your beautiful cabinetry? ever have any of your clients call to inform you that you have burned there home down due to shotty installation practices? all of that said of course you know as a homeowner/builder with no permit you can do anything you'd like. So that all said "let the build begin" would you like to discuss the application of exterior finishes as well....................................Jim, Your "Moral" Contractor

I'm a smokaholic. I own many grills/smokers. Trying to cut back but having a hard time. <br />I'm a shriner and ride on a competition motorcycle drill team. <br />Family comes first!

Posted 05 September 2009 - 02:49 PM

Thanks for moving this thread, Mikey!

Seems that plywood covered with granite would have a very hard time combusting since the granite would absorb the heat that it takes to create a fire. Also it removes air from the equation. We all know that it takes 3 ingredients for fire. Oxygen, heat and fuel. If you remove any one of these 3 necessary items, there cannot be fire. Could it singe? Maybe. I have a wood stove in my basement that has ceramic tile for the surround. I do have hardi behind it but of course there is wood framing right behind that. My stove can get to very high temps. No problems. the other side is I used hardi behind my ceramic shower surround so moisture is not a problem with the product. Outdoor use of hardi is fine. YOu are covering it with stucco or stone or something else so moisture isn't the concern.

Just for the fun of it, I will place a small piece of exterior plywood right on the side of my grill between the firebox and frame and see what happens. If it lights, which I doubt, it will prove one way or the other. Keep in mind, heat RISES. Take a look at my shelves on my ceramic.... <------------ They are not burned up and they are right beside my cooker firebox. Wood handle on my gas grill too. Last grill had wood shelves right next to the firebox. Didn't burn.

I, too, am thinking of and starting to plan my outdoor kitchen. Of course I will have it inspected, as I always do things the right way, by the law, and will follow local building codes... ;>D

Great point Sink, Im sure the hardie will brown from heat as well as the ply, but starting fire I doubt highly as you pointed out, My ducane sits on my patio a foot or so out from the wall and i'll be dammed if my nice white stucco isn't all browned above the grill lid and the patio roof as well from hot smoke, but as we all know heat rises and when my ducane is cleaning at 900 degrees the side walls of my grill are cool to the touch, but it's a cart not a built in.....................The Build Continues..........jim

I used Cement board... No fires, almost waterproof. no bugs
Costs alittle more, but no bigger than most outdoor kitchens are it is a small increase in price

DUROCK cement board is moisture and mold resistant and does not deteriorate, swell, soften, decay, delaminate, or disintegrate in the presence of water, making it the perfect choice for baths, showers, kitchens, and laundry rooms. It is also suitable for exterior applications, including fences, mobile home skirting, agricultural buildings, garage wainscoting, and exterior finishes.

The board is non-combustible and can be used in a variety of fire-rated designs. Its low thermaland hygrometric expansion help prevent finish cracking.

DUROCK cement board includes a 30-year transferable warranty for interior applications and a 10-year transferable warranty for exterior applications.

I don't claim to be an expert in construction, nor to I claim to be an expert in physics. I don't play either on TV. I didn't even stay at a Holiday Inn Express last night. But I have enough of a combination of physics knowledge and common sense to know that some of the arguments being made here have basic flawed logic. 2 points in particular need some clarification:

1. Assuming that a layer of "X" will protect a flammable substrate (where X is granite, tile, concrete, metal, etc). As stated, granite will absorb the heat around it. This does not mean said heat magically disappears. The heat it absorbs will be radiated to all surfaces around it until thermal equilibrium is again achieved. This means that the flammable substrate (plywood in this discussion) will be still be exposed to the heat, though potentially in a slower and/or more spread out fashion than if directly exposed.

2. The theory that "I placed a piece of plywood next to or in contact with my grill for 'y' period of time and it didn't catch on fire so it will never burn" is flawed as well. Combustion doesn't have a time requirement, and can slowly occur over a period of months/years. As stated, oxygen, heat, and fuel are required for combustion. But when discussing the combustibility of wood, one must consider how much heat is required. One must also consider that this minimum temperature changes with many variables. One of these variables is the moisture content - the moisture content in wood must be reduced/eliminated for the wood to burn, and this process can occur very slowly over time. As it does, the minimum temperature for ignition gets lower and lower.

Again, I'm not stating what can or cannot be used in outdoor kitchen (or any other) construction. I just know enough to know that there's too much that I don't know. As such, I'll err on the side of caution for myself and my family.

Some interesting reading that is somewhat Big Green Egg-specific, but still very much applicable and on topic can be found here: